The invention relates to a cementless endoprosthesis primarily for hip joints comprising a screw spindle to be screwed into the medullary bone canal, a conical neck mechanically connected with the proximal end of the screw spindle, the axis of the neck angularly arranged with respect to the axis of the screw spindle.
An endoprosthesis of this kind has been published 1948 by McBride. Because the core of the screw spindle must transmit extreme forces the diamter must be as great as possible. McBride has proposed to drill a bore into the medullary bone canal to cut a thread and to screw in the screw spindle containing the neck for a head of the prosthesis. The thread of the spindle should be anchored in the cancellous bone structure to provide a fixation of the spindle. However, it has been recognized, that this art of arthroplasty has a plurality of deficiencies. The medullary bone canal is not straight but antecurved. The bone is necessarily weakened in some areas by drilling the straight bore. Just in these weakened areas of the bone the best anchoring effect of the spindle is gained. Because of the curvature of the bone canal there remain broad surfaces of the canal with which the thread of the spindle does not come into engagement. Because the proximal femur portion opens in a bell-shaped manner, the spindle stands free in this area. The thread cuts into the cortex at the distal end of the spindle and because the cortex structure is much stronger than the cancellous structure the prosthesis is supported substantially at its distal end. The result is a permanent proximal relief and weakening of the proximal load suspension structures and the biomechanical response is an atrophy of the bone structures which increases from proximal to distal. Further deficiencies consist in that the height of the thread flank could only be small in order not to weaken the core of the spindle so that the thread provides only a small form-fit engagement and last but not least in that the spindle cannot be firmly held against rotation.
Another type of endoprosthesis known as "Spotorno Prosthesis" which is used nowadays consists of a stem tapering to its distal end and having oblong cross-sections somewhat adapted to the shape of the medullary canal of the bone. The stem is beaten axially into the canal and wedges itself therein. A plurality of longitudinal ribs are provided at the broad side faces of the proximal portion of the stem to prevent any rotation of the stem. However, also this kind of endoprosthesis is not satisfactory because press-fit anchoring takes place only at its distal end in the cortex of the bone canal and the results are the same as mentioned above.